The present invention relates to a vehicular automatic transmission for changing power transmission paths by controlling operation of hydraulic gearshift means.
Automatic transmission for automobiles are arranged to shift
Automatic transmission for automobiles are arranged to shift gears automatically dependent on running conditions of a motor vehicle to achieve desired vehicle running characteristics. It is customary to provide a gearshift map composed of upshifting and downshifting curves for each gear position, the curves being established in relation to the vehicle speed and the engine power output, and to control the automatic transmission to shift the gears according to the gearshift map dependent on the running conditions as indicated on the gearshift map. One example of such gear shifting control is disclosed in Japanese Laid-Open Patent Publication No.61-189354, for example.
One type of automatic transmission includes a power transmission means comprising a plurality of power transmission paths (e.g., a plurality of gear trains), a plurality of gearshift means (e.g., a plurality of hydraulically operated clutches) for selecting the power transmission paths, and a control means (e.g., a hydraulic pressure control valve) for controlling operation of the gearshift means. When a running conditions of a motor vehicle, as indicated on a gearshift map, moves across an upshifting or downshifting line, a gearshift command is produced to effect an upshift or downshift, and a solenoid valve is operated based on the gearshift command to control operation of the hydraulic pressure control valve to engage one of the hydraulically operated clutches. The power transmission path through a certain gear train associated with the engaged clutch is now selected to effect a gearshift.
The gear ratio of previous gear position (which is established by a power transmission path or gear train that has been selected until a gearshift command is issued), and the gear ratio of a next gear position (which is established by a power transmission path or gear train selected by the gearshift command) are different from each other. Therefore, it is important that the transmission be controlled not to produce a gearshift shock and a gearshift delay when a gearshift is made from the previous gear position to the subsequent gear position.
It has been proposed to connect an accumulator to the hydraulically operated clutches or gearshift means to lessen a change in the torque applied for engaging a next-gear-position clutch smoothly when making a gearshift, or to provide a hydraulic pressure release valve (such as an orifice control valve, a timing valve, or the like) for releasing the hydraulic pressure from a previous-gear-position clutch at a rate corresponding to the rate at which the hydraulic pressure in a next-gear-position clutch is increased, so that the hydraulic clutch pressure is controlled depending on the engine power output (see Japanese Laid-Open Patent Publication No. 60-211152, for example).
When the accelerator pedal is depressed and the transmission is shifted down (a "power-on/downshift" mode corresponding to a kickdown) or when the accelerator pedal is released and the transmission is shifted up (a "power-off/upshift" mode), a smooth and shock-free gearshift can be effected by engaging the next-gear-position clutch when the rotational speeds of the input and output shafts thereof are synchronized. The rotational speed of the engine is varied in the direction in which the rotational speeds of the input and output shafts of the next-gear-position clutch are synchronized by releasing the previous-gear-position clutch.
A difference in gear ratio between the gear position achieved when the previous-gear-position clutch is engaged and the gear ratio of the gear position achieved when the next-gear-position clutch is engaged is constant, a change produced in the rotational speed of the engine when a gear-shift is made varies depending on the engine rotational speed at the time of starting the gearshift. For example, when a gearshift is made from the third gear position down to the second gear position while the engine rotational speed is 1000 RPM, the engine rotational speed increases by 500 RPM to 1500 RPM. When the same gearshift is effected while the engine rotational speed is 2000 RPM, the engine rotational speed increases by 1000 RPM up to 3000 RPM.
With the absolute value of the change in the rotational engine speed being thus varied, the time consumed until the rotational speeds of the input and output shaft of the next-gear-position clutch are synchronized is also varied. According to the conventional gearshift control, since the hydraulic pressure is released from the previous-gear-position clutch and the hydraulic pressure is increased in the next-gear-position clutch at a constant rate, it has been difficult to effect smooth gearshifts for all engine rotational speeds.